Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Wed Nov 04 2020

Valid 00Z Thu Nov 05 2020 - 00Z Sun Nov 08 2020


...Western U.S....
A wet pattern will persist across the Northwest through Thursday
as a cold front sinks slowly south across the region.  A shortwave
amplifying off of the Northwest coast late in the day is expected
to help tap deeper moisture -- supporting an additional round of
organized heavier precipitation along the orographically favored
regions of western Washington and Oregon.  Snow levels are
expected to lower some on Thursday; however, the threat for heavy
snowfall accumulations is expected to remain confined to the high
peaks of the northern Cascades.

Models continue to show the previously noted shortwave digging
south, with a closed low developing off of the Oregon coast
Thursday night, before dropping into Northern California on
Friday.  This will bring the focus for better organized
precipitation and heavier accumulations farther south across
Oregon into Northern California.  While this will bring colder air
as well, the overall trend of the models has been warmer, with
lighter snowfall accumulations expected through Friday along the
southern Cascades into the northwestern California coastal ranges
and the northern Sierra.  Meanwhile, a lingering low-to-mid level
baroclinic zone interacting with an influx of deeper moisture is
expected to support organized precipitation from northeastern
Oregon to western Montana.  Here too, the guidance has trended
warmer, with heavy accumulations outside of the higher elevations
of the northwestern Montana ranges not expected.

On Saturday, models show the upper low beginning to weaken and
transition to an open wave as it moves east across California into
the Great Basin and Southwest.  Snows are expected to continue
across the coastal ranges of northwestern California and the
Sierra into early Saturday before developing along the Transverse
and coastal ranges of Southern California later in the day. 
Again, any heavy accumulations are expected to remain confined to
the higher elevations.  Expect light accumulations to spread east
across the mountains of northern and central Nevada through early
Saturday and then along the Utah ranges and San Juans as the upper
system moves east late in the period.

For Days 1-3, the probability of significant icing (0.25-inch or
more) is less than 10 percent.

Pereira